A typical phase control ballast for a high intensity discharge (hid) lamp comprises a reactor and bilaterally conducting switching means which are connected in circuit with the lamp and an alternating voltage source such as the conventional 60 hertz ac supply. The switching means ordinarily will be a solid state switch such as a triac. A control circuit triggers the triac on with some phase delay at each half cycle of the source voltage. Ideally for a circuit in which lamp, reactor and switch are connected in series across the source a reactor is chosen that will limit the current to a value providing slightly better than rated power or wattage input to the lamp when line voltage is at its lower limit and the voltage drop across the lamp is at the nominal value. Under these circumstances the function of the control circuit is to delay or retard in phase the triggering on of the switch whenever the line voltage is above its lower limit. By so doing the current build-up during the remainder of the half cycle is limited and the wattage input into the lamp is regulated.
In general it is desired to cope with supply line variations in voltage of .+-.10%. As for lamp voltage variations, they can occur both as a result of manufacturing tolerances and, depending upon the kind of lamp, as a result of aging. Some high pressure sodium vapor lamps may experience a rise in arc voltage drop of as much as 50% over the life of the lamp which may exceed 20,000 hours. The extent to which the control circuit can maintain the wattage input into the lamp constant notwithstanding line and lamp voltage variations is a measure of its quality and effectiveness.
When line voltage applied to a reactor in series with a discharge lamp is increased a small amount, power into the lamp increases drastically. Therefore in using an open loop series phase control approach to regulate against line voltage variations, a linear increase in line voltage requires a supra-linear increase in the retard of the phase angle. Up to the present, a low cost supra-linear open loop feedback scheme which is precise and does not change with temperature or the inevitable variations in the parameters of circuit components was unavailable.